Agricultural drainage ditches, their biological importance and functioning

B I O L O G I C A L C O N S E RVAT I O N

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available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/biocon

Review

Agricultural drainage ditches, their biological importance and functioning Irina Herzon*, Juha Helenius Department of Applied Biology, University of Helsinki, Latokartanonkaari 5-7, 00014, Finland

A R T I C L E I N F O

A B S T R A C T

Article history:

We reviewed studies on the biological state of agricultural drainage ditches in the temper-

Received 26 October 2007

ate and boreal zones of the Northern Hemisphere. We looked at the relative importance of

Received in revised form

ditches for farmland biota as compared to that of other habitats, and assessed the degree to

3 March 2008

which biological communities of ditches contribute to the provisioning of ecosystem ser-

Accepted 10 March 2008

vices. We evaluated impacts pertaining to replacement of open drains by subsurface drain-

Available online 8 May 2008

age, removal of main ditches, rehabilitation of old drainage systems, and maintenance of ditches. Most ditches support species also common elsewhere. Whenever comprehensive

Keywords:

surveys were conducted, ditches were shown to provide valuable wet vegetated non-

Ecosystem services

cropped habitats to both aquatic and terrestrial taxa, supply food resources lacking in

Farmland biodiversity

otherwise dry and intensively managed cropland, and perform connectivity functions

Literature review

within a wider landscape. Regionally ditches were shown to harbour rare species or species

Management

not found presently in other farmland habitats. Some functions of drainage ditches, such

Subsurface drainage

as regulating water flow and nutrient retention, are likely to depend on the composition and structure of biological communities of ditches, though the issue remains poorly explored. The biggest threat to the quality of ditch networks as ecosystems is presented by a severe runoff from the fields, management in disregard of a habitat value of ditches, and removal of ditches. Ó 2008 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methodology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biological communities of ditches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Plants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Invertebrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Fish and amphibians . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Birds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Mammals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

* Corresponding author: Tel.: +358 9191 58318 (work), +358 405330946 (mobile); fax: +358 9 191 58582. E-mail addresses: [email protected] (I. Herzon), [email protected] (J. Helenius). 0006-3207/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2008.03.005

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4.

5. 6. 7.

1.

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Functioning of ditches as ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Water purification, nutrient cycling and erosion control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Pollination and pest control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Habitat provision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Science and recreation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recent changes in ditch extent and quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Managing ditches for multiple functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction

Drainage in agriculture refers to the artificial removal of water from land and is employed in the reclamation of wetlands, the prevention of erosion, as a concomitant of irrigation, and for the general enhancement of agricultural land-use by improving the efficiency and timing of farming operations. Large-scale drainage has a major impact on the farming practice by affecting the scale of cultivation, the types of crops planted, water and nutrient regimes, and the structure of fields. The impact also extends beyond the fields, affecting hydrology over catchments and landscape structure. Until very recently the side-effects of drainage works had been generally overlooked. The change was driven by a dramatic reduction in natural wetlands across Europe and North America, concerns over the contamination of waters draining from agricultural land, severe floods due to fast drainage into rivers, and by dramatic decline in farmland biodiversity (EEA, 1996). In Austria and Denmark, for example, land drainage was cited as ‘‘probably the single most important measure which has adversely affected the landscape (loss of wetlands, small scale structures in the landscape), the biodiversity and the hydrological cycle’’ (EEA, 1996). There are several ways by which drainage practices affect functioning of agroecosystems and wider landscapes. Originally, drainage meant above all gain of cultivated land at the expense of wetlands and peatland. In Europe and the USA, where the primary drainage works have mainly been completed, drainage is addressed by its role in general water and chemical transfer over the whole catchment (EEA, 1996; Ohio State University, 1998). Further loss of ditches to subsurface piping and land consolidation remains a matter of controversy in some countries (Hietala-Koivu et al., 2004). In the countries of Central and East Europe, that joined the EU in 2004, rehabilitation of the existing drainage systems causes concerns over the loss of spontaneously re-naturalised wetlands within farmland (Dzalbe and Busmanis, 2007). There is a widespread need for reconciliation of management of ditches for their drainage functions with the support of biodiversity and associated ecosystem services (Bradbury and Kirby, 2006). While the non-cropped habitats within fields are generally regarded as crucial for wildlife, the ecological value of ditches is less clear. As Twisk et al. (2000) pointed out research has so far focused on terrestrial flora and fauna of hedgerows and arable crop margins, more than to aquatic field margins. A

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number of studies have demonstrated that streamside vegetation has a positive effect on native fauna disproportionate to its limited extent (review in Bennett et al., 2006). Relatively high biodiversity in such areas can be attributed to cool moist conditions, high productivity, complex habitat, and within cropland, lower disturbance. Wet patches within a welldrained cropland were suggested to be ‘‘keystone structures’’ (Tews et al., 2004). Ditches can be expected to differ from larger water courses and bodies in several aspects (cf. Søndergaard et al., 2005 for ponds). Being linear elements with a high edge ratio ditches are subjected to an intensive exchange of matter and organisms from the surrounding terrestrial matrix. Most of the ditches are likely to be relatively shallow with marked fluctuations in water levels and a higher probability of drying out during summer. Finally, ditches are regularly managed for efficient drainage. The typical surface system consists of field drains, field ditches, a main collection ditch, and an outlet. As a habitat a (drainage) ditch includes a permanent or temporary water course, the bottom substrate, vegetated or bare bank slope, and a vegetated margin. The widths of the three parts vary greatly: from 20-cm wide field drains conducting water during short periods of snow melt and heavy rains and overgrown with grass, to 10 m-wide permanent water courses with complex vegetation. Many of the latter are channelized streams and small rivers. In fields drained by ditches parallel drains run every 20–50 m resulting in 200–500 m of non-cropped strips covering 2–5% of field area. Fields are commonly surrounded by small ditches in some regions of Europe. Subsurface drainage systems consist of conduits, a submain, a main – as subsurface pipes, and an outlet. It is installed to drain a new field or replaces ditches. Efficiency of the drainage system, surface vs. subsurface, will depend on soil type, field slope, rainfall pattern, and types of crops to be grown. We reviewed the literature on the biological state of agricultural drainage ditches. A specific objective was to search for evidence on the role of the biological communities in the overall functioning of ditches in farmland under the framework of the Millennium Ecosystem Assessment (Millennium Ecosystem Assessment, 2005). We further evaluated impacts pertaining to replacement of open drains by subsurface drainage, removal of main ditches, rehabilitation of drainage in countries of central and Eastern Europe. We briefly outlined a strategy for managing ditches for multiple functions, and challenges for further research.

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2.

Methodology

We targeted the review to boreal and temperate biogeographical zones of the Northern Hemisphere. We excluded southern regions with drastically different climatic conditions and prevalence of irrigation ditches. We systematically searched scientific publications from 1971 to present from: ISI Web of Knowledge, PubMed, Wiley, Agricola, JSTOR, Directory of Open Access Journals. We used such entries as ditch and farmland, drainage and agriculture, ditch and species, ran searches for similar papers whenever available, and screened the citations in the papers for earlier studies. We additionally searched technical reports, project overviews, and statistical data via Google and from relevant official sites. Finally, we sent personal queries to some authors and relevant policy makers. A total of over 1000 publications and unpublished reports were reviewed, over 300 of which were regarded as relevant to the review objectives. Herein we excluded references to purely observational studies, but the complete list is available from the authors. We categorised the results by the main species groups and by the topics on the functioning of ditches, potentially dependent on biological communities, recent changes in ditch occurrence and quality, legal status of ditches, and management.

3.

Biological communities of ditches

Few studies provided an assessment of the ditch communities involving multiple taxa or different trophic levels (exceptions being Armitage et al., 2003; Twisk et al., 2003; Davies et al., 2008). Similarly, in most of the botanic studies, occurrences of species are not described across the whole ditch ecosystem but are restricted to either terrestrial or aquatic parts (exceptions being Bouldin et al., 2004; Dajdok and Wuczyn´ski, 2005). The only national review is available from Defra (2002), and we omit the further examples and references provided therein. Many of the species occurring in ditches require different combinations of a variety of features, including water, soil and vegetation, and many more are likely to benefit from ditches only during some time of the breeding cycle. Ditch characteristics quoted as primarily affecting species assemblage composition are a hydrocycle, which determines hydroperiod, depth, flow, and chemical composition of water. These in turn depend on the ditch type and size, surrounding farming system and natural habitats, and management regime.

3.1.

Plants

Higher plants are the best studied taxon of ditches and their banks. Examples of regional inventories of the ditch aquatic vegetation are available from the UK (Defra, 2002; Goulder, 2007), Denmark (Baattrup-Pedersen and Riis, 2003), The Netherlands (Twisk et al., 2003), Poland (Dajdok and Wuczyn´ski, 2005), and the USA (Bouldin et al., 2004). The aquatic plants recorded in ditches probably largely originated from a predrainage marshland flora that has since been lost in many regions (Goulder, 2007). Ditch bank vegetation is extensively studied in, for example, Finland (Tarmi et al., 2002), The Netherlands (Blomqvist

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et al., 2003; Manhoudt et al., 2005), and the UK (Milsom et al., 2004). While ditch banks support considerably higher plant species diversity in comparison to crops and sown field margin, most of the species are also common elsewhere. The vegetation cover is comprised mainly of species that are indicative of high nutrient content of soil and tolerance to common herbicides, that is, characteristics of noxious weeds. However, in The Netherlands many previously common plant species of wetlands, grasslands and dry hayfields are currently found mainly in ditch banks with many having a national or international conservation status (Blomqvist et al., 2003). The adjacent land-use additionally affects the species composition in ditch banks. In the UK, ditches bordering arable land were shown to have a great abundance of P. australis, which is less tolerant of grazing (T. Milsom, cited in Bradbury and Kirby, 2006). Generally more plant species are found in ditch banks within grasslands than crop fields (Tarmi et al., 2002), while in wetland landscapes (fenlands and valleys), ditch floras can be outstanding in diversity (Biggs et al., 2007). There is a generally assumed positive relationship between the diversity and complexity of vegetation and the diversity of animal taxa that rely on vegetation (Higler and Verdonschot, 1989; Armitage et al., 2003). However, it has been questioned whether a similar pattern applies for aquatic vegetation and fauna, especially if a large variety of taxa is considered (Declerck et al., 2005). The direction and strength of such relationships seem to be taxon-dependent in ponds (Oertli et al., 2002) but it has not been studied for ditches.

3.2.

Invertebrates

Some general aspects pertaining to the invertebrate assemblages in ditches have been described. First of all, all stages of hydroseral succession of plants support distinct invertebrate communities and, consequently, different species will be associated with ditches of different sizes and stages of vegetation succession (examples in Defra, 2002; Scheffer et al., 1984; review in Bradbury and Kirby, 2006). Taxonomically related species were shown to have a spatially similar distribution pattern in ditches, and temporal changes in the macroinvertebrate fauna during the summer appear to be at least as prominent as the spatial heterogeneity (Scheffer et al., 1984). Due to the restricted dispersal of many invertebrates, proximity of ditches to each other and to larger waterbodies may be crucial in their community assemblage (e.g. Rouquette and Thompson, 2005). Finally, the quality of water and its oxygen status were influential in determining macroinvertebrate communities (Clare and Edwards, 1983; Foster et al., 1990). Among groups common in drainage ditches are freshwater gastropods, including some rare species (Watson and Ormerod, 2004), larvae of most of the common hoverflies, water beetles, large branchiopods, and some benthic taxa. Benthos seemed to receive less attention in the studies of ditches (exception in Richards et al., 1993), and far more macroinvertebrate species were collected within the water column and on plants than in the benthos (Clare and Edwards, 1983). Most ditches seem to be hydrologically unsuitable for the breeding of dragonflies but in the USA ditches attracted a unique set of transient species periodically emigrating from breeding areas (Bried and Ervin, 2005). Some of the ditches and channelized

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streams remain a valuable habitat for crayfish species (Jay and Holdich in the UK, 1981; Na¨reaho et al., 2006 for Finland), though preferred conditions of non-polluted water and overhanging vegetation are rarely found in farmland. A relatively large research effort has been targeted at invertebrate communities of ditch banks, for which a character of the terrestrial vegetation was shown to be critical. The presence of specific host and nectar plants determines possible occurrence of many invertebrates, and stability of the vegetation is a major prerequisite for safe overwintering. A general character of a ditch bank will be decisive for some groups: for example, the fauna of Hemiptera from shaded buffer zones was found to be different from that of open sunny ones (Norrdahl and Yletyinen, 2007), and presence of windbreaks facilitates movements of butterflies (Kuussaari et al., 2007). Other invertebrate groups, such as bumblebees and spiders, also derive only an indirect value from ditches and are associated with ditch margins (e.g. ¨ berg et al., 2007). A Salix group commonly Sepp et al., 2004; O growing along larger ditches is known to be the key foraging resource for bumblebee females emerging in early spring (Tera¨s, 1985). Banks of ditches running across fields are wet, often exposed to wind, and have a low diversity of a few dominant plant species, which means they provide less suitable habitat than margins along forest edges or dry roads (Kuussaari et al., 2007 and references for butterflies). Interestingly, in Finland, a width of a ditch slope, rather than a margin width, was positively related to the number and abundance of butterflies (Helio¨la¨ and Kuussaari, 2008). Margins surrounded by arable crop monocultures are preferred by eurytopic species with a high tolerance for disturbance or having weeds and grasses as larval host plants (Pywell et al., 2004; Kuussaari et al., 2007). Only few species were shown to associate with reeds and rushes (Bradbury and Kirby, 2006). In the UK, the highest density of Lycaena phlaeas was recorded along ditches, even though these habitats were relatively rare. Its formerly common host plant became confined to ditch banks in arable landscapes (Leon-Cortes et al., 2000).

3.3.

Fish and amphibians

Larger ditches with permanent water supply may have a variety of fish species (examples in Defra, 2002). In Finland, some ditches – former streams – still provide spawning habitat for a freshwater population of Salmo trutta (Na¨reaho et al., 2006). Frogs readily use ditches for breeding and feeding (e.g. Piha et al., 2007 on Rana temporaria). Though generally absent from intensively farmed areas and ditches dry in the spring, frogs and newts (Triturus cristatus and Triturus vulgaris) can benefit from ditches that dry up later in summer since this excludes predatory fish. Ditches were shown to facilitate movements of frogs across otherwise hostile agricultural landscapes (Reh and Seitz, 1990; Mazerolle, 2004), and contributed positively to the genetic diversity of populations of Rana temporaria (Vos and Chardon, 1998). In the USA, road-side ditches are considered as a high-quality habitat for Bufo americanus and Pseudacris triseriata, so that populations of these species are shown to largely depend nowadays on the retention of ditches in farmland (Rustigian et al., 2003).

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3.4.

Birds

Species richness of farmland birds relates well with the level of heterogeneity of farmland, to which ditches in some regions contribute considerably (Tryjanowski, 2000; Vepsa¨la¨inen et al., 2005; Herzon and O’Hara, 2007). According to Bradbury and Kirby (2006), the ditch as an ecosystem provides a variety of resources for farmland birds: damp soil for probing species, permanent water to provide aquatic invertebrates, bare or sparsely vegetated ground to improve access to benthic and soil invertebrates, rank emergent vegetation for nesting, and bush and tree groups to provide nesting and singing posts. Among birds closely associated with drainage ditches in farmland are Acrocephalus scirpaceus, A. palustris, A. schoenobaenus, and Emberiza schoeniclus – all in need of persistent stands of reeds and tall emergent plants. Populations of these species suffered from drainage, and within intensively farmed landscapes they rely heavily upon the vegetation of road sides and drainage ditches (Catsadorakis, 1997). A study in Poland (Surmacki, 2005) revealed some species-specific differences in the importance of ditches among alternative habitats of small and large marshes with A. palustris showing a clear preference for ditches. Other passerine species also extensively use ditches and their margins, though for them the relative importance of a margin with or without a ditch is less clear. In Poland, margins with and without ditches had similar numbers of breeding species (Dajdok and Wuczyn´ski, 2005). In the UK, the presence of a ditch doubled the number of species registered in farmland (Arnold, 1983). When both hedges and ditches were present, Prunella modularis held more territories than with only hedges. However, according to the same author a ditch without a hedge seemed to be of little value, except to Alauda arvensis. The latter species relies on safety of the grassy margins along open ditches, and its association with ditches running across an field area was shown for Sweden (Berg and Pa¨rt, 1994), Finland (Piha et al., 2003), the Baltic states (Herzon and O’Hara, 2007). In Finland, the abundance of A. arvensis decreased less in a farmland area drained by surface ditches than in one with subsurface drainage (Mehta¨la¨ et al., 1985). The population crash of E. hortulana in Finland was in part attributed to a strong decline in the length of ditches with bushes and solitary trees within farmland (Vepsa¨la¨inen et al., 2005). Notably, retaining some trees does not mitigate the loss of the ditch. A considerable number of other farmland birds were shown to benefit from ditch presence (Berg and Pa¨rt, 1994 for Sweden; Mehta¨la¨ et al., 1985; Valkama and Currie, 1999; Jobin et al., 2001 for Canada; Tiainen et al., 2005 for Finland; review of Bradbury and Kirby, 2006 for the UK). Waders were proved to benefit from re-wetting narrow channels formerly used for drainage in the UK (Eglington et al., 2008). Use of ditches by birds other than for nesting during the breeding season and outside it is generally overlooked. This might in part explain the lack of any connection in community characteristics with the presence of a ditch in some studies (Benoıˆt et al., 2001; Dajdok and Wuczyn´ski, 2005). Only Arnold (1983) studied use of ditches by birds in winter and showed that the bigger the ditch the more it was used as a source of food and shelter.

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Waders use a gently sloping bank for probing, during migration and breeding, massive emergence of semi-aquatic invertebrates, and presence of ditch banks with aquatic insects, worms or molluscs attract numerous feeding passerine species to wet habitats in farmland (reviewed by Bradbury and Kirby, 2006). Vegetated margins are generally regarded as patches rich in invertebrate and seed food for a variety of birds (Vickery et al., 2002). A number of predatory birds are dependent on ditches as sources of prey. Numbers of Falco tinnunculus declined in Finland following the subsurface draining (Valkama et al., 1995). Finally, the presence and usability of main ditches tended to increase survival of Bucephala clangula by providing connectivity in Finnish farmland (Po¨ysa¨ and Paasivaara, 2006).

3.5.

Mammals

Permanent water in well vegetated ditches is a prerequisite for the occurrence of Agricola terrestris, Neomys fodiens, and Lutra lutra (Defra, 2002). All grass-dominated habitats in Europe are inhabited by Apodemus sylvaticus and Microtus arvalis (Heroldova´ et al., 2007 in Czech Republic). In the USA, abundance and species richness of small mammals were greater in buffer zones than in pastures, and especially high near streams (Chapman and Ribic, 2002). Increasing an area of linear habitats was reported to result in a more balanced diversity of the small mammalian community (Butet et al., 2006). It is highly possible, though not ascertained, that the linear elements act as corridors in farmland, allowing movements that would otherwise not occur (Mauritzen et al., 1999). Increased width of a buffer zone was shown to reduce the frequency with which M. arvalis uses a neighbouring crop field (Yletyinen and Norrdahl, 2008). In Lithuania, as much as 30% of the total population of Castor biver inhabit drainage ditches (Lamsodis et al., 2006). Avoidance of intensively farmed agricultural areas by Microchiroptera is attributed to the reduction in insect densities (Wickramasinghe et al., 2004 for the UK).

4.

Functioning of ditches as ecosystems

According to a fundamental classification of ecosystem services (Millennium Ecosystem Assessment, 2005), the major regulating functions of the ditch network within cultivated catchments include: (i) acceleration of transfer of water and soluble nutrients from the fields, (ii) water retention and nutrient recycling within the ditches; (iii) uptake and release of phosphorus and nitrogen by vegetation, (iv) mitigation of herbicides in vegetation and sediment; (v) modifying erosion rate and transfer of soil-bound nutrients, and (vi) supporting pollination and pest control functions. Vegetation of ditches and margins can potentially be used as a source of fodder and biomass, which is an example of the provisioning functions. Provision of habitat as such is a supporting function. Cultural functions include amenity value of agricultural landscapes as well as enabling scientific research and recreation. The ecosystem functioning of ditches can be best appreciated considering the fact that in the modern agricultural landscape they, together with subsurface pipes, constitute the main component of the water transfer network, which

Fig. 1 – Conceptual framework for impacts of drainage on processes in and structure of an agricultural catchment; in italics are examples of associated costs and benefits.

redistributes water within farmland and links fields with receiving aquatic systems. In many highly simplified and homogenised production landscapes ditches often represent the only network of non-cropped elements. Therefore, impact of drainage is ultimately related to the processes of water and nutrient flows in and structural characteristics of the drained catchment (Fig. 1).

4.1. Water purification, nutrient cycling and erosion control In most cases subsurface drainage is more effective than open drains (e.g., Turtola and Paajanen, 1995; Carluer and Marsily, 2004), and allows the increment of cropland through removal of ditches, reduction in maintenance costs, and more efficient working of enlarged fields (Haataja and Peltola, 2001; Myyra¨, 2006). Though improved drainage has been extensively studied in Europe and the USA (e.g., Birgand et al., 2007; Salo and Turtola, 2006 and references), there are few assessments comparing subsurface drainage and with open drains over agricultural catchments (review for the USA in Ohio State University, 1998). Subsurface drainage affects overall water redistribution, so that a larger portion of water leaves the field through the subsurface system. Due to water filtration through the soil, loss of sediment and soil-bound nutrients, such as phosphorus, decreases. On the negative side, a subsurface drainage system enhances movement of water-soluble nitrogen to surface waters due to the increased rate of discharge and probably nitrogen mineralization (Skaggs et al., 2005). In most cases, the sum of environmental benefits is assumed to overweigh drawbacks (Ohio State University, 1998), though the trade-off is specific for different soils. On heavy clay soils in Finland, greater water discharge after improvements to subsurface drainage led to a 70% increase in nitrogen leaching either without or with only small reduction in a total phosphorus loss (Turtola and Paajanen, 1995; Salo and Turtola, 2006). Other functions performed by ditches are lost once the subsurface drainage system is installed or main collection

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ditches are piped. The main known mechanisms of removal of agricultural pollutants from surface water are re-infiltration in marginal and aquatic vegetation, in sediment and through enhanced degradation by sun (Beltman et al., 2004; Mankin et al., 2007; Mulholland et al., 2008). Vegetation inside ditches and in banks was shown to enhance mitigation of herbicides and some insecticides to their almost total removal (Moore et al., 2001; Borin et al., 2004). Many pesticides have to be water-soluble in order to be efficient, and therefore their pathway is likely to be similar to that of soluble nitrogen. With subsurface drainage they are discharged into larger water courses, bypassing the degradation potential of the ditch vegetation. Vegetation also reduces water flow velocity in ditches, which further enhances agrochemical retention and degradation (Beltman et al., 2004; Borin et al., 2004). Compared with removal of agrochemicals in surface runoff, there are relatively few possible mitigation measures for drainflow, so some countries introduced regulatory disadvantage for subsurface drainage by putting restrictions on pesticides for use on drained land (Reichenberger et al., 2007). The effectiveness of terrestrial vegetation in buffer zones in reduction of erosion and nutrient loss greatly vary (Dorioz et al., 2006; Lovell and Sullivan, 2006; Mayer et al., 2007). Notably, buffers were shown to be not as effective in nutrient removal in fields with subsurface drainage (Osborne and Kovacic, 1993). Once open drains are removed, alternative practices, such as discharging drain pipes into additionally constructed wetlands running parallel to the water course, may be needed to control non-point agricultural pollution. Attempts were made to rank ditches according to their ability to reduce excess nutrients and pesticides based on their vegetation character. Bouldin et al. (2004) suggested that a sufficient macrophyte/water contact is fundamental in mitigation of agricultural contaminants but the mere presence of grassy buffers rendered the most explanation of the reduced water pollution for all ditch types. Legacherie et al. (2006) confirmed that the ditches’ performance was highly variable. No studies are available on the functioning of small field drains, with or without permanent water. Having dense bed vegetation, they are likely to act similarly to narrow buffers in filtering runoff waters but also in releasing phosphorus during the dormant stage. Their proximity to the source of chemical runoff is likely to be beneficial in immediate removal of certain contaminants.

4.2.

Pollination and pest control

Enhancement of pollination and biological control are attributed to all non-cropped habitats within farmland (reviewed in Marshall and Moonen, 2002). Ditch margins present an extensive network of such strip habitats, up to 10% the field area in open drain systems. The decline in semi-natural vegetation, including that of ditch banks, combined with the restricted foraging radius of many invertebrates, was cited as key factors in the loss of pollinating species and potential pollinator limitation in modern agricultural landscapes (Walther-Hellwig and Frankl, 2000). Many studies worked on a hypothesis about an enhanced biological control in heterogeneous agrolandscapes with a dense network of non-cropped habitats but the consensus has not been reached (Tscharntke et al., 2007).

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Semi-natural vegetation within farmland performs also a number of negative functions by providing host plants and overwintering sites for pests, and facilitating weed dispersal into crops.

4.3.

Habitat provision

The agronomic value of ditches and the high costs of converting to subsurface drainage or filling larger ditches make ditches a relatively permanent non-cropped structure, so that ditch value in habitat and community restoration can be relatively high. For example, establishment of unsprayed strips in fields without any connection to pre-existing ditch banks was shown to be of little use to butterflies (De Snoo, 1995). Vegetation of old ditch banks was more diverse than that of recently established ones (Geertsema and Sprangers, 2002). However, there is evidence from The Netherlands that the current seed bank of ditch margins is already depleted and its potential for restoration is minimal. On most banks high biomass and competition appear to further hamper germination (Blomqvist et al., 2003). The value of ditches and their banks in proving habitats and refugia as compared to other habitats remains an open question. The relative importance of ditches is likely to depend on the availability of other non-cropped and wet structures within a farmland area, such as natural (streams, wetlands) and other artificial (ponds) habitats. Some studies provide data on the relative occurrence and abundance of species in ditches as compared to alternative habitats, but most of them deal only with margins (Leon-Cortes et al., 2000; Benoıˆt et al., 2001; Armitage et al., 2003; Surmacki, 2005; Williams et al., 2004; Biggs et al., 2007; Kuussaari et al., 2008). Most ditches, especially small ones and those with temporary water, are arguably habitats of lower quality than larger and more stable water environments, and therefore they are no substitute for ponds or streams and lakes. Similarly, margins can be a poor substitute for semi-natural grasslands. Creating and conserving small elements functioning as population sinks might not be wise when it is possible to enlarge the source areas with the same effort (Kleijn and van Langevelde, 2006). However, sinks may play a critical role in maintaining effective populations across an agricultural landscape by providing alternative habitat types (for example, fish-free habitat for amphibians) or movement corridors, by decreasing population recovery time following hostile events such as drought (Rustigian et al., 2003), and by contributing to the stability of source populations (Foppen et al., 2000). Geertsema and Sprangers (2002) showed that the spatial clustering of ditch banks under management agreements has higher importance for population survival probability of plant species than the habitat quality and disturbance frequency. In Finland, those species of butterflies of the semi-natural habitats that frequently used margins declined less severely than those which never did so (Helio¨la¨ and Kuussaari, 2008). It is often uncertain, in which areas and to what extent linear landscape features support viable populations and serve as refugia, and therefore should be protected. In some intensively managed arable landscapes, such elements are the only remaining non-cropped or wet habitats. The contribution of

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small non-cropped elements, even species-poor, to the total species pool in homogeneous agricultural landscapes may be relatively more substantial than in respective diverse ones (Bengtsson et al., 2005; Herzon and O’Hara, 2007). Therefore, fields with open ditches in regions where they are few and land-use is most intensive, may be of a greater value than in regions with smaller fields, more grassland, and abundant edges. Ditches may harbour a suite of plants and animals not found presently in other farmland habitats (Boutin et al., 2003; Armitage et al., 2003; Williams et al., 2004; De Bie et al., 2008). In the UK, ditches, most of which were seasonal, were the least species-rich habitat, but supported uncommon species not recorded in other types of water bodies (Williams et al., 2004). Also findings on temporary ponds (Collinson et al., 1995; Oertli et al., 2002; Nicolet et al., 2004) suggest that even small and ephemeral water bodies can contribute to regional biodiversity. A well established productivity-diversity relationship (Wright et al., 2006) precludes expectation of high levels of plant species diversity ditch banks within farmland. However, the common and abundant species are often the ones that provide most of the ecological services (Balvanera et al., 2005), including maintenance of species at higher trophic levels (reviewed by Marshall and Moonen, 2002). Wet areas are likely to be hotspots for abundant emergent insects – an important resource for farmland birds (Bradbury and Kirby, 2006). Even initially common species may decline as rapidly as some rare species (e.g., Lycaena phlaeas in the UK, Leon-Cortes et al., 2000). Fauna of freshwaters may be particularly vulnerable, and in North America the extinction rates among

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freshwater fauna were as high as among the fauna of tropical forests (Ricciardi and Rasmussen, 1999).

4.4.

Science and recreation

The scientific value of ditches is not explored though may be similar to that of other small-scale wet elements such as temporary pools (Blaustein and Schwartz, 2001) and ponds and pools (De Meester et al., 2005). Ditches are regionally abundant, represent a gradient of ecological states in terms of hydroperiod and nutrient concentration, are well delineated aquatic ecosystems in a terrestrial landscape, support relatively simple communities, and are subject to strong anthropogenic influence. These are characteristics of a potentially good model system for research in ecology, evolutionary biology and conservation biology (De Meester et al., 2005). Ditch networks provide a suitable research setting on questions of connectivity for both aquatic and terrestrial organisms. Enhancement of species considered ‘‘interesting’’ appeals to people pursuing hunting, fishing or bird and butterfly watching. In some landscapes, ditch margins provide walking routes in farmland. A high level of support for buffers by farmers and other residents may mean that the buffers provide considerable amenity values in the agricultural landscape (Sullivan et al., 2004 for the USA).

5.

Recent changes in ditch extent and quality

In most European countries, drained lands represent a considerable proportion of farmland (Table 1). In the USA and Canada 25% of agricultural soils are artificially drained

Table 1 – Agricultural land drainage in Europe (only some northern and western countries are presented Country Czech Denmark Estonia France Finland Germany Hungary Latvia Lithuania Netherlands Poland Sweden

Drained lands, ha

%c

450,000 1,440,000 420,000 2,500,000 1,280,000 >4,900,000 2,320,000 2,620,000 2,620,000 3,000,000 4,205,000 1,100,000

12 55 48 10 57 29 50 100e NAe 100e 25 37

Surface drainage NA 0 18,000 200,000 600,000 100,000d 0 0 0 1,400,000 1,500,000 0

% of all drained NA 0 7 8 34 2 0 0 0 47 36 0

Comment

A

B C D

Sources for drainage statistics are from international commission on irrigation and drainage (ICID, data from 1990 to 2003)a, and for the agricultural land are from FAOb for 1999–2003. Comments: A. Total length of drainage ditches 17,669 km (Agricultural Research Centre, Estonia). B. Subsurface drained area is 4900,000 ha (incl. the former eastern Germany). Figures for West Germany: area with open drains = 100,000 ha and with drain pipes = 1500,000 ha. C. Since 1900, new drainage construction practically stopped; 320,000 ha are for irrigation. D. Some 53,000 km of open ditches. Works mostly took place in the 1950s, and mainly accomplished by 1990. a b c d e

http://drainage.montpellier.cemagref.fr/drainage_systems.php. http://www.fao.org/es/ESS/census/wcares/. Of the total agricultural land area. Only for the former Western Germany. Drained area encompasses virtually all agricultural land but also land-used for other purposes.

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(www.pca.state.mn.us/publications/presentations/lakepepinschraderpart10706.pdf). Where field drains were once in use, their replacement with subsurface piping has been largely finalised (e.g., Sweden). Only in Finland, The Netherlands and Poland a substantial area of fields remains drained by open field drains (Table 1). In Finland, the national drainage plan aims at further halving the arable area still drained by open ditches by the year 2020 (Drainage Centre, 2002). The dramatic loss of open drains with their margins was cited as one of the key aspects of the decrease in agrolandscape diversity in the post-war period (Hietala-Koivu, 2002). Since the middle of the 20th century subsurface drainage has gradually become more important also in The Netherlands. State support for drainage has been considerable in all countries, and remains so in some (a tax for financing the drainage agencies in Italy, P. Rossi, LIPU, pers. comm.). Throughout Europe, main collection ditches have been being increasingly replaced by pipes, often with state support, due to consolidation of holdings and reparceling. Removal of the remaining ditches is most profitable in areas already under intensive production, where the ecological value of such non-cropped elements is the highest (see above). This invariably means the loss of grassy margins, and hence often of the only remaining uncropped area within a crop. For example, in Denmark the extent of field boundaries was reduced by 30% within preceding 100 years, and their character changed once ditches and dikes were replaced by mere contact zones between two crop types (Moller, 1983). Drainage maintenance presents a challenge in the new EU member states of Central and Eastern Europe. Abandonment of farmland and lack of drainage management in some regions led to the spontaneous re-establishment of the previously drained wetlands, and subsequent improvement in biodiversity within farmland. A considerable portion of the national rural development plans in Estonia, Latvia, and Poland is allocated for the re-instating of neglected drainage systems. This process has already been reported to have a damaging impact on valuable wet grassland habitats (Keenleyside et al., 2006). The biggest threat to the quality of ditches as ecosystems is presented by a severe runoff from the crop fields. Under eutrophic conditions, phytoplankton and floating plants increase, followed by a severe loss of submerged plants and regression of emergent vegetation (Janse et al., 1998). A few macroinvertebrates or vertebrates can survive such conditions (Twisk et al., 2000; Marklund et al., 2001; Scha¨fer et al., 2007). Several European aquatic species of streams were driven close to extinction as a result (Riis and Sand-Jensen, 2001). In the UK, ditches were considered to be intermediate between ponds and streams in the pesticide-sensitive invertebrate species (Biggs et al., 2007). The structure and function of stream communities within farmland was shown to be impaired by pesticides in several European countries (Scha¨fer et al., 2007). Terrestrial vegetation associated with ditch banks is similarly affected by nutrient and pesticide allocation. In a study of large-scale changes in the abundance of common higher plant species across Britain in the 1980s and 1990s, Smart et al. (2005) showed that linear habitats in lowland Britain displayed trait changes consistent with a secondary succession due to systemic nutrient enrichment in both lowland and up-

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land environments. A long-term reduction in plant species richness in ditch banks was reported from The Netherlands (Blomqvist et al., 2003) and Finland (Kuussaari et al., 2008). Farming type can be expected to affect communities of the ditches. The terrestrial biota of margins is more diverse on farms under organic management than under conventional (reviews by Bengtsson et al., 2005), thought the effects of fertilizer applications on vegetation was shown to be far more severe than those of herbicides (Kleijn et al., 1997). Ditches within conventional crop fields and on heavy soils tend to be of especially poor biodiversity value (Manhoudt et al., 2007). The effects on aquatic vegetation have not been reported. Presence of non-disturbed natural ecosystems in upstream reaches of ditches improves the quality of polluted downstream reaches (Scha¨fer et al., 2007).

6.

Managing ditches for multiple functions

The relative values of ditches in draining land, control of water flow and chemical transfer, and as a wildlife habitat are likely to vary greatly regionally and even locally. In areas of prime agricultural production the key ditches for efficient drainage should be managed primarily for this purpose, though their removal should be discouraged. Even in these cases incorporation of some wildlife-friendly practices might nonetheless be possible (Defra, 2002). In areas where drainage needs and production have declined and landscape values of farmland gained prominence, or where some ditches have lost drainage capacity, management should be relaxed and ditches reprofiled. Even when ditches are surrounded by intensively managed fields and their quality tends to be low, their biological value can be improved by proper management (Foster et al., 1990). There is a general consensus that a dynamic management regime, providing vegetation at different stages of succession, is the best alternative. In practice, it requires rotational de-silting and vegetation clearance in ditches across a wider landscape, retention of vegetation and flow refuges, and disturbance timed outside spring and early summer (Foster et al., 1990; Defra, 2002; Lancaster, 2000; Twisk et al., 2000). When species of particular conservation value are found in ditches, management can take these into account. Certain management options beneficial for wildlife will also promote retention and removal of agrochemicals. Periodic dredging is usually required to remove excess vegetation to restore efficient draining, return nutrients from silt onto the fields, and recreate the habitat. Harvesting of plant biomass from inside the ditch and buffer zones reduces the amount of phosphorus released in the dormant season (Osborne and Kovacic, 1993). Allowing grassed ditches within fields ameliorates the otherwise weak herbicide-retention ability of ditches (Legacherie et al., 2006).Vegetated and meandering stretches for slowing water flow may be effective, while constructed wetlands downstream are a promising tool for mitigating pesticide inputs and retaining nutrients (De Laney, 1995). In some cases nature can do the job: waters in drain stretches dammed by beavers had 30–40% lower concentrations of dissolved inorganic nitrogen and phosphorus than in stretches above the dams (Lamsodis et al., 2006).

xxx (rare or interesting spp.)

xxx xx (soil capture over extended time) xxx xxx xxx xxx

xx (complex vegetation, plankton diversity)

x (productivity effect) xx (complex ditch and vegetation profiles xx (complex ditch and vegetation profiles)

Supporting Cultural

Science and recreation

Decomposition of organic matter Erosion prevention Pollination of crops Pest and disease control Provision of habitat Landscape amenity

Mitigation of herbicides

Logistical in collecting and transporting Regulating optimal performance under annual variability Adequate management of vegetation (regular removal of biomass but encouraging diverse vegetation to capture nutrients, removal of sediment with minimum disturbance) Sufficient vegetation/water contact Slowing the water flow Diversity of aquatic species Establishment of sufficiently wide filtration strips, diversified vegetation Diversified vegetation Diversified vegetation with a minimised ratio of pest and disease host plants Adequate management, minimised disturbance Irregular attractive feature Adequate management, diverse vegetation As above Biomass for feed and energy Drainage of water and water retention Uptake and release of phosphorus and nitrogen Provisioning Regulating

Challenges for management Examples

Applying the evaluation framework proposed by the Millennium Ecosystem Assessment to real landscapes and discrete land-use practices presents a practical challenge. Further development of the service framework to make it more operational for land managers and planners is an important step forward (Wallace, 2007). However, research on establishing links of the community composition of species (potentially) inhabiting drainage ditches with the functioning of ditches within a specific regional setting has rarely been done. Most surveys of ditch biota are localised and provide a snapshot picture of one or a maximum of two taxonomic groups. Research from East European countries is practically non-existent. Ditches nonetheless feature as a habitat in a considerable number of studies on a variety of species groups. What is often lacking is an assessment of the relative value of ditches among alternative habitats, their importance as multiple resources for mobile species, and the role of vegetation and faunal composition in the overall functioning of drainage ditches. Based on results from other ecosystems, we can suggest a number of aspects of dependence between services and community as in Table 2. A diversity of vegetation types and plant forms is likely to increase the physical contact with chemicals and intensify their reduction, while a diversity of ecological profiles may capture nutrients across a maximum possible vegetative period. Similarly, diversity of plankton taxa may accelerate the denitrification function within a ditch, while in margins the diversity of terrestrial plants, including flowering plants, may enhance soil retention and nutrient uptake. On the other hand, the presence of species of particular ecological profiles most efficient in performing these functions is likely to be more important than diversity of species as such (Cardinale et al., 2006), though the relative input of species may vary depending on climatic conditions. The diversity of ditch biota under the most severe impact from pollutants is

Table 2 – Ecosystem services of drainage ditches and their plausible dependence on biological communities

Synthesis

Dependence on community diversity

Some spontaneously re-established wetlands in Central and Eastern Europe can be retained as sedimentation pools. It is expected that land drainage in the EU will generally decrease (EEA, 2007). Already by now overdraining may have happened in Lithuania, where it led to the consequent need or irrigation (http://www.fao.org/nr/water/aquastat/regions/ fussr/index7.stm), and possibly elsewhere in Europe (Reichenberger et al., 2007). The risk may increase with climate change if, as predicted, there is a change in precipitation patterns with fewer but more intense rainfall events (EEA, 2007). In Austria and Germany, the drainage is no longer subsidised by the government and programmes to re-establish natural hydrological features have been started (EEA, 2007). In Finland, the decision on granting drainage investment, additional to agronomic needs, is based on possible environmental benefits of subsurface draining, such as erosion control and pollution prevention. However, negative effects of accelerated nitrogen leaching are not given sufficient weight in the case against drainage. In spite of the acknowledged adverse impacts on biodiversity, no practical solutions are suggested (Haataja and Peltola, 2001).

7.

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inevitably greatly reduced and the resistance of the remaining species is critical in supporting community functioning. Whether massive declines and directional changes in aquatic and terrestrial taxa in ditches translate into population changes for species of higher trophic levels is not known. Further research on the functioning of ditches could be directed at: (i) further elucidating the functioning of ditches in removal of chemicals and eroded soil particles (e.g., efficiency in nutrient binding by a network of ditches as compared to a single sedimentation pond) and provision of other services (e.g., provision of quality habitat and its desirable extent); (ii) identifying species and species groups which have the key role in performing the services – ecosystem service providers (sensu Kremen, 2005); (iii) determining aspects of community structure that influence functioning and may reduce its variability; (iv) measuring the spatio-temporal scale at which services are provided and determining factors that contribute to the increase of these scales. The above could be used in developing a methodology on ranking ditches by their relative importance in providing various services, for example, drainage performance versus pollution mitigation versus habitat provision. Finally, assessment and management of drainage systems for multiple functions will have to be based on the whole catchment or landscape level, since most of the functions reviewed here operate on scales large than individual farms and are dependent on the landscape context.

Acknowledgements The review was financially supported by Finnish Cultural Foundation (to IH). Technical assistance of Tiina Hovi and Silvia Budaviciute, advice from Ma Maohua and Melita Zecvojinovic, thoughtful revision of English by Frederick Stoddard, and valuable comments of two reviewers are highly appreciated.

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